Method of making protected galvanized steel sheeting

ABSTRACT

A method is proposed for applying a weather-resistant, oxidative-polymerizable, hot-melt, monomer-modified, polyester resin coating to galvanized steel sheets to produce heat-formable protected metal sheets.

United States Patent Summers et al.

METHOD OF MAKING PROTECTED GALVANIZED STEEL SHEETING Inventors: John E. Summers, Pittsburgh, Pa.;

Gerhard Rose, Neukirchen,

Germany Assignee: Robertson Bauelemente G.m.b.H.,

Germany Filed: June 7, 1974 Appl. No.: 477,203

Related U.S. Application Data Continuation-impart of Ser. No. 402,710, Oct. 2. 1973, abandoned, which is a continuation of Ser. No. 249,414, May 1, 1972, abandoned.

U.S. Cl. l48/6.l6; 148/62; 117/75 Int. Cl. C23! 7/26 Field of Search... ll7/l32 A, l6l K, 75, 105.2,

[4 June 24, 1975 [56] References Cited UNITED STATES PATENTS 2.525. I07 l0/l950 Whiting et al. l48/6. 16 2,692,840 l0/l954 Bell l48/6.l6 2,798,017 7/1957 Lesser l48/6.l6

Primary Examiner-Harry J. Gwinnell Attorney, Agent, or Firm-Harry B. Keck; George E. Manias [57] ABSTRACT A method is proposed for applying a weatherresistant, oxidative-polymerizable, hot-melt, monomer-modified, polyester resin coating to galvanized steel sheets to produce heat-formable protected metal sheets.

5 Claims, 5 Drawing Figures F: U DUGGI METHOD OF MAKING PROTECTED GALVANIZED STEEL SHEETING CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 402,710 filed Oct. 2, 1973, now abandoned, which was a continuation of application Ser. No. 249,414 filed May 1, I972 and abandoned on Oct. 2, I973.

BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates to a method of coating galvanized steel sheets to produce useful, post-formable, weather-resistant, color-coated building sheets for the construction of buildings and building panels.

2. Description of the Prior Art Weather-resistant, oxidative-polymerizable, hotmelt, monomer-modified, polyester resin coatings have been applied to asphalt-impregnated, asbestoslaminated, zinc-coated steel sheets of the type described in U.S. Pat. No. 2073,3134. Such weatherresistant, oxidative-polymerizable, hot-melt, monomermodified, polyester resin coatings have not been applied directly to galvanized steel sheets in the past. Indeed such coatings would not reliably adhere directly to galvanized steel sheets.

A number of primers including wash primers have been applied to galvanized steel sheets. See Protective Coatings for Metals, Burns and Bradley, ACS Monograph Series No. I29, Reinhold Publishing Corporation, I955, Pages 4l4-4I6.

Application of a weather-resistant, oxidative-polymerizable, hot-melt, monomer-modified, polyester resin coating to galvanized steel sheets has not been heretofore carried out.

SUMMARY OF THE INVENTION According to the present invention, galvanized steel sheet is preheated to a critical temperature in the range of 60-100C, without other pretreatment. The preheated galvanized steel sheet is coated with a dispersion of a wash primer in a solvent. The wash primer is applied, preferably by roller coating, over at least one surface of the preheated galvanized steel sheet. The primer-coated, galvanized steel sheet is transported from the primer coating rolls by rapidly rising hot gases which provide the transportation impetus and also serve to flash vaporize the solvent from the wash primer dispersion.

The wash primer coated galvanized steel sheet is reheated to a temperature of 60l00C and a coating of weather-resistant, oxidative-polymerizable, hot-melt, monomer-modified, polyester resin coating is applied over the primer coating by roller coating. The hot-melt coating is applied to the coating rolls at a temperature which is above the softening temperature of the coating resin and is in the range of I80-220C. Immediately after the hot-melt resin coating application is completed, the hot-melt resin coated sheets are quenched in a water bath to cool the hot-melt coating to a temperature which is at least 50C below the softening temperature of the hot-melt resin, i.e., below about 50C.

The resulting coated sheets are post-formable, i.e., the resulting coated sheets can be rolled or bent into useful profiled shapes without adversely affecting the adhesion or appearance of the outer coat. Such postforming should occur either immediately or within a few days following the application of the outer coat while the outer coat is post-formable. The oxidative-polymerizable character of the outer coat introduces a transition of the outer coat within a few days as a result of oxidation of the oxidative-polymerizable resin, whereby the resin is converted to a cross-linked, thermoset material.

Application of weather-resisting coatings to galvanized steel sheets can be accomplished by passivating the galvanized coating with phosphatizing and/or chromating pretreatments whereby the thus pretreated galvanized steel sheets will accept virtually any primer coating. It is an object of the present invention to produce coated galvanized steel sheets without requiring such extensive passivating pretreatment of the galvanized coating.

It is a further object to provide a weather-resistant, post-formable, galvanized steel sheet having a weatherresistant, oxidative-polymerizable, hot-melt, monomermodified, polyester resin coating.

BRIEF DESCRIPTION OF THE DRAWING(S) FIG. 1 is a cross-section schematic illustration of typical apparatus employed to carry out the present process;

FIGS. 2, 3, and 4 are cross-sectional views of the product of the present invention in various forms.

FIG. 5 is a cross-sectional illustration on an exaggerated scale illustrating the roller coating application of the primer coating according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Wash primer systems are known in the art. See Protective Coatings for Metals, supra. These primer systems include a polyvinylbutyral resin, zinc tetroxy chromate solvents and aqueous phosphoric acid.

The primer systems also include suitable volatile solvents such as isopropyl alcohol; modifying resins which may be melamine, phenolic, vinyl ethers, phenoxies, epoxies, alkyds and in general any compatable filmforming resin. The systems also may include pigments, settling agents, leveling agents and other useful additives.

In accordance with the present invention, the primer system includes an abnormally high quality of volatile solvent, such as methylethylketone. Customarily, wash primers are applied to galvanized steel by spraying, brushing or swabbing. According to the present invention, the wash primer is applied by roller coating. The abnormally high amount of volatile solvent in the system permits rapid spreading of the dispersion over the preheated galvanized steel sheet in the brief time between application of the dispersion and the flash volatilization of the solvent. The wash primer dispersion has a viscosity of 10 to I5 seconds DIN-Cup No. 4. The volatile non-aqueous solvents constitute more than per cent by weight of the wash primer dispersion. In the preferred embodiment, hot gases rise up beneath the freshly coated galvanized steel sheet and serve to flash vaporize the solvent, to maintain a cushion between the coated sheet and the subjacent apparatus and also to reheat the primer coated sheet prior to application of the weather-resistant, oxidative-polymerizable, hotmelt, monomer-modified polyester resin coating. Preferably, the hot gases are combustion gases obtained from burning hydrocarbon fuel with air.

Referring to FIG. 1, a coil of galvanized steel is uncoiled and delivered to a guillotine shear II where suitable lengths are cut. The individual sheets 12 are delivered along a production line sequentially through a preheat zone [3, primer coating rollers 14, a solvent flash zone 15, a preheater 16, hot-melt coating rollers I7 and a quench bath 18 to a finished sheet collection zone 19.

The galvanized steel coil 10 preferably has a steel thickness of about 28 to about l4 gauge and a galvanized coating of about 0.3 to 1.5 ounces per square foot (about 0.l5 to 0.75 ounces per square foot on each side). Hot dip galvanized coatings are contemplated, although the zinc coating may be electrocoated or deposited from vaporized zinc, for example. Suitable conveyor arrangements are provided, but not shown, for delivering the individual sheets 12 through the equipment [1, l3, l4, 15, 16, 17, 18, 19.

The individual sheets 12 of galvanized steel which are cut off at the guillotine shear 11 may range up to about 40 feet in length and normally may be as short as 3 or 4 feet. The girth of the galvanized steel sheets similarly may range from about 1 foot to about 5 feet. Wider sheets are contemplated but are not presently available in the steel industry. Individual sheet 12a is shown passing through a preheating zone 13 wherein rapidly rising hot gases from burner jets serve to preheat the sheet 120 to an appropriate preheat temperature in the range of 60IOOC. The off-gases from the pretreatment zone 13 are collected and exhausted into the atmosphere as flue gases through an appropriate stack 21. The sheet 12a at a temperature of 60-IOOC is delivered into the nip of a pair of coating rollers 14 which are supplied with a wash primer dispersion in solvent. The rollers 14 are shown in FIG. 5 with a sheet of galvanized steel therebetween. The top coating roller 14a is vertically aligned with the bottom coating roller 14b. A doctor roller 22 is maintained in controlled proximity to the perimeter of the top coating roller 14a whereby a reservoir 23 of the wash primer dispersion is maintained between the rollers 14a, 22. The preheated galvanized steel sheet 12b receives a supply of the wash primer dispersion as a film 24. The preheated galvanized steel sheet 12b is at a temperature which is above the flash temperature of the wash primer solvent so that the wash primer solvent quickly evaporates through a flash vaporization, leaving behind the relatively nonvolatile components of the wash primer dispersion.

The bottom coating roller of 14b of FIG. 3, also is provided with a doctor roller which maintains a reservoir 26 of the wash primer dispersion. The spacing be tween the doctor roller and the perimeter of the coating roller 14b is fixed to establish the thickness of the wash primer film 28 which is applied to the undersurface of the galvanized steel sheet 12b. The films 24, 28 may have different thicknesses or may have the same thickness. In some cases, it is not necessary to apply the wash primer to both surfaces and in this instance, one of the two coatings 24, 28 can be eliminated. The coating rollers 14a, 14b are preferably formed from a resilient substance such as polyurethane elastomer, rubber, other resilient plastic materials.

The sheet 12b having a wet coating of wash primer dispersion is delivered by means of a gas stream from left to right through the solvent flashing zone 15. A hot rising gas stream is delivered from a pump 29 through a conduit 30 into a plenum chamber 31 whence the hot gases are released upwardly through a perforated plate 32. The upwardly rising hot gases support the sheet 12b out of contact with the structure and serve to carry away the volatilized solvent. The hot gases and solvent vapors are withdrawn through a stack 33. The primercoated sheet is delivered from the solvent vaporization zone 15 into a sheet reheating zone 16. Within the sheet reheating zone 16, the sheet 12c is reheated to a temperature of 60l05C and also additional volatile solvents are withdrawn from the primer film and the primer film is set or cured on the galvanized sheet surface. The heat for the sheet reheating zone 16 is supplied by means of combustion gases which are delivered upwardly through a stack 34 and preferably are collected by the pump 29 for recirculation through the system as the carrier gas in the solvent vaporization zone 15. The reheated, primer-coated saturated sheets 120 are delivered to the nip of hot-melt coating rollers 17 which apply a weather-resistant, oxidative-polymerizable, hot-melt, monomer-modified, polyester resin coating to one or both sides of the reheated primercoated sheets 12d. The coating composition is maintained as a liquid at a temperature of l220C prior to and during application. The resulting coated sheets with the hot-melt coating in a still-molten condition are delivered into a water bath 18 where the hot-melt coating congeals; non-tacky sheets 12 e are withdrawn from the water tank 18 with the hot-melt coating cooled to a temperature which is at least 50C below the softening temperature of the hot-melt coating. The coated sheets l2e are collected in a stack 19 as flat sheets. Alternatively, the coated sheets 122 may be delivered directly to a roll forming line to be corrugated into useful construction sheets.

The hot-melt coating of this invention is described as being oxidative-polymerizable and this means that the coating will undergo certain oxidative-polymerization reactions, when exposed to the atmosphere, so that the thermoplastic character of the outer coating is shortlived and the surface of the outer coating becomes nonfusible.

By controlling the composition of the outer coating, the duration of its thermoplastic character may be regulated. These coated sheets remain post-formable so long as the outer coatings remain thermoplastic, normally from several days to several months, depending upon the requirements of the product.

In FIGS. 2, 3 and 4, three different variations of the present color-coated galvanized steel product are demonstrated. In each of the three variations, certain numerals are the same: 35, the steel core; 36, the galvanized zinc coating; 37, the wash primer coating; and 38, the hot-melt, oxidative-polymerizable, monomermodified, polyester resin outer coat. In FIG. 2 the protected metal sheet includes a galvanized coating 36 on both sides of the steel core 35; a wash primer 37 on both surfaces; a hot-melt outer coat 38 on both surfaces. An alternative product of FIG. 3 has the galvanized coating 36 applied to both surfaces of the steel core 35 but the primer coat 37 and hot-melt outer coat 38 are applied only to one surface. Such products would be useful in building installations wherein a galvanized steel sheet requires a weather-resistant coating on the outer surface but is essentially protected from atmospheric influences on the inner surface.

The alternative sheeting of FIG. 4 has a galvanized coating 36 on both sides of the steel core 35 and has a wash primer 37 on both of the galvanized coatings 36. The hot-melt outer coat 38 is applied only to one primer-coated surface. Such products would have a utility in a single skin building product having one side exposed to the atmosphere and having the other side exposed to the interior to permit the building owner to apply a subsequent coating or paint or other protective material to the primer-coated inner surface, identified by the numeral 39.

The three products shown in FIG. 2 all have in common the exposed surface treatment, i.e., a steel core 35 having a galvanized layer 36, a wash primer 37 and a hot-melt, weather-resistant outer coating 38. The thickness of the wash primer 37 is from about 5 to about 20 microns in the final product. The thickness of the weather-resistant hot-melt outer coat 38 in the panel product is from about 50 to 200 microns.

The speed of moving the sheets 12a, 12b, 12c, 12d, l2e through the processing line of FIG. 1 can be adjusted inversely according to the thickness of the steel in the coil [0. That is, 14 gauge galvanized steel sheets require a longer residence than 24 gauge galvanized sheets for the reason that they require longer time in the preheat zone 13 and in the reheat zone 16 and in the water cooler 18 for the required heat transfer. Throughput of about to 200 feet per minute is anticipated.

PRIMER COMPOSITIONS A typical wash primer for the present product has the following composition:

COMPONENT A (DISPERSION) Zinc Tetroxy Chromate 7.8% Polyvinylbutyral Resin 5.2% Melamine Resin l.7% lsopropyl Alcohol 50.4% Toluene l4.9% COMPONENT B (SOLUTION) Aqueous Phosphoric Acid (85%) 3.4% lsopropyl Alcohol ll.6-l4.fi% Water 2.0-S,0%

COMPONENT A and COMPONENT B are admixed to produce a more or less conventional wash primer composition. However, in accordance with this invention, the mixture of COMPONENT A and COMPO- NENT B is further blended with a volatile organic solvent, e.g., methylethylketone, in a ratio of about 1:] to provide desired lowered viscosity. The boiling temperature of methylethylketone is about 68C.

The dilute dispersion of wash primer can be readily applied by roller coating over the surface of galvanized steel sheets. Because the sheets are preheated, the solution of wash primer is rapidly dried through flash vaporization of the volatile solvent. Rapidly rising hot gases complete the solvent volatilization and drying of the wash primer coat.

THE HOT-MELT OUTER COAT The typical hot-melt oxidative-polymerizable outer coat contains an oxidative-polymerizable resinous vehicle formed by the esterification of unsaturated aliphatic acids (such as tall oil fatty acids, oleic acid, dehydrated castor oil, tung oil, linseed oil), styrene, alkyl methacrylates of acrylates, dicarboxylic acids such as phthalic anhydride, triols such as trimethylol ethane, and the like.

Alternatively the hot-melt, oxidative-polymerizable polyester resin outer coat contains a saturated polyester resin to which minor amounts of unsaturated aliphatic acids are added without accompanying esterification; e.g., the outer coat may contain a saturated polyester of phthalic anhydride, adipic acid, glycol, trimethylol ethane which is physically mixed with 2 per cent by weight of dehydrated castor oil to achieve the desired oxidative-polymerizable character of the outer coating.

In addition to the polyester resin vehicle, the coating compositions typically contain high-melting point waxes; wetting agents; driers such as cobalt naphthenate, pigments, inert fillers such as crushed limestone, talc, short asbestos fibers; tinting pastes.

The hot-melt outer coat normally has a penetration value of 2-l5 (preferably 6-12) measured at 77F (100 grams, 5 seconds); and has a Stormer viscosity of l525 seconds measured at 400F (300 grams, I00 revolutions). The coating has a softening temperature, ASTM ball-and-ring of l05l20C. The hot-melt outer coat remains thermoplastic for a sufficient period to permit post-forming of the protected steel sheets. The loss of thermoplasticity occurs through oxidative-polymerization of the polyester coating which depends upon exposure of the coating to air. The duration of thermo-plasticity (post-formability) will range from several days to several months.

During its application, the hot-melt outer coating is maintained in a molten condition at a temperature of about l80220C and the primer-coated galvanized steel sheet is coated at a preheated condition at about 60l05C. The hot-melt outer coating is applied at a thickness of about 50 to 200 microns and is quickly chilled by quenching in water to a temperature of about 50C below the fusion temperature of the coating, preferably to a temperature of about 50C.

We claim:

1. The method of making a protected metal sheet having a post-formable, oxidative-polymerizable polyester resin outer coat, comprising:

a. preheating a sheet of galvanized steel to a tempe rature of 60 to C;

b. applying a film of wash primer dispersion in volatile solvent to at least one surface of the preheated sheet;

c. concurrently (i) volatilizing the solvent of the wash primer dispersion film; (ii) curing the wash primer film; and (iii) reheating the sheet to a temperature of 60 to [05C;

d. applying, at a temperature of 180 to 220C, a

weather-resistant, hot-melt, oxidative-polymerizable polyester resin coating having a softening temperature of I05 to C, ball-and-ring, to at least one wash primer-coated surface of the said sheet which is at a temperature of 60 to [05C;

e. cooling the hot-melt, resin-coated sheet to set the hot-melt resin coating at least 50C below the softening temperature of the resin coating.

2. The method of claim 1 wherein the coating of wash primer is applied by roller coating to the galvanized sheets.

3. The method of claim 1 wherein the said sheet of galvanized steel has not received any phosphatizing or chromating pretreatment.

4. The method of claim 1 wherein the said wash primer dispersion contains at least 70 per cent by weight of volatile solvents.

5. The method of claim 1 wherein the said wash Itzlrimer dispersion has a viscosity of 10 to 15 DIN, Cup 

1. THE METHOD OF MAKING A PROTECTED METAL SHEET HAVING A POST-FORMABLE, OXIDATIVE-POLYMERIZABLE POLYESTER RESIN OUTER COAT, COMPRISING: A. PREHEATING A SHEET OF GALVANIZED STEEL TO A TEMPERATURE OF 60* TO 100*C; B. APPLYING A FILM OF WASH PRIMER DISPERSION IN VOLATILE SOLVENT TO AT LEAST ONE SURFACE OF THE PREHEATED SHEET; C. CONCURRENTLY (I) VOLATILIZING THE SOLVENT OF THE WASH PRIMER DISPERSION FILM; (II) CURING THE WASH PRIMER FILM; AND (III) REHEATING THE SHEET TO A TEMPERATURE OF 60* TO 105*C; D. APPLYING AT A TEMPERATURE OF 180= TO 220*C, A WEATHERRESISTANT, HOT-MELT, OXIDATIVE-POLYMERIZABLE POLYESTER RESIN COATING HAVING A SOFTENING TEMPERATURE OF 105* TO 120*C, BALL-AND-RING, TO AT LEAST ONE WASH PRIMER-COATED
 2. The method of claim 1 wherein the coating of wash primer is applied by roller coating to the galvanized sheets.
 3. The method of claim 1 wherein the said sheet of galvanized steel has not received any phosphatizing or chromating pretreatment.
 4. The method of claim 1 wherein the said wash primer dispersion contains at least 70 per cent by weight of volatile solvents.
 5. The method of claim 1 wherein the said wash primer dispersion has a viscosity of 10 to 15 DIN, Cup No.
 4. 